Advances in absorbent article technology have stimulated the search for absorbent (often superabsorbent) materials with desirable properties such as high absorption, high storage capacity and high gel and mechanical strength.
The absorbent materials may include two or more layers such as liquid acquisition layers, storage layers and distribution layers.
In order to obtain good liquid acquisition capacity it is important that the absorbent material has a high momentaneous liquid acquisition capacity. Open, bulky structures with large capillaries have a high momentaneous liquid acquisition capacity and examples of such materials are cellulosic fluff pulp of thermomechanic or chemithermomechanic (CTMP) type, chemically stiffened cellulosic fibres fibers, waddings of synthetic fibers and porous foam materials.
In order to obtain a good liquid storage capacity it is common that the absorbent structure contains superabsorbent materials. Superabsorbent materials are crosslinked polymers with the capacity to absorb liquid many times their own weight. Organic materials suitable for use as a superabsorbent material can include natural materials such as polysaccharides (particularly modified polysaccharides such as CMC: carboxymethylcellulose), polypeptides and the like, as well as synthetic materials such as synthetic hydrogel polymers. Such hydrogel polymers include, for example, polyacrylates (particularly alkali metal salts of polyacrylic acids), polyacrylamides, polyvinyl alcohol, polyacrylamides, polyvinyl pyridines, and the like. Other suitable polymers include polyvinylamine, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers and mixtures thereof. The absorption mechanism of such superabsorbents is thought to be based on the fact that the polymer chain contains a plurality of charged groups, which make it possible for the polymer network to absorb aqueous liquids by means of osmotic forces.
The superabsorbent material in an absorbent structure, i.e. a diaper core, is often in the form of small particles, which are arranged and contained in a fibrous matrix. The fibrous matrix usually includes cellulosic fluff pulp of thermomechanic, chemical or chemithermomechanical type, but a certain amount of synthetic fibers are also common.
One problem with absorbent structures containing superabsorbent material is that it is difficult to distribute and maintain the superabsorbent material in the desired location in the absorbent structure, both during storage and during use of the article. Another problem with absorbent structures containing superabsorbent material is so-called gel blocking. This problem occurs by the fact that the liquid-containing superabsorbent particles swell and form a gel. The gel blocks the liquid transport and gives rise to an accumulation of liquid in certain portions of the absorbent structure while other portions of the structure become more-or-less non-utilized.
In order to obtain a superabsorbent material with high mechanical strength, the degree of crosslinking of the polymer is crucial. The more crosslinking in a polymeric structure, the more the mechanical strength increases. However, a high degree of crosslinking within a structure restricts the swelling capacity of the material, and highly-crosslinked superabsorbent materials are brittle and break easily. The performance of superabsorbent materials in different applications is highly dependent on the elastic modulus, the resistance to fracture and the water absorbance capacity among other properties. These properties are strongly affected by the degree of crosslinking. It has been shown that for polyacrylic acid (PAA) the equilibrium degree of swelling decreases and the elastic modulus increases with an increasing degree of crosslinking, as expected from theory.
Absorbent web composites including a superabsorbent polymer component and fibrous material, such as cellulose fibers and processes for their production are known.
An example of this type of material is described in US 2003/0111163, which describes a process for making an absorbent fibrous web composite including a stable and controllable dispersion of superabsorbent polymer. Two polymer precursors, for example, acrylic acid or methacrylic acid, are added in separate stages to form a superabsorbing polymer on or in a pre-formed fibrous web, which includes a plurality of hydrophilic fibers, e.g. microfibrillar cellulose or microcrystalline cellulose.
Similarly, US 2003/0111774 describes a process for making an absorbent fibrous composite nonwoven web including e.g. superabsorbent polymers and plurality of hydrophilic fibers. The polymerization of the superabsorbent polymer is integrated into the process of forming the absorbent composite nonwoven web.
EP 1 207 914 further discloses an absorbent structure including an open-cell foam structure wherein the pore walls of the structure include a liquid-storing material, e.g. polyacrylate. The absorbent structure is characterized in that the pores of the foam structure contain hydrophilic fibers, e.g. cellulose fibers, at which at least the main part of the hydrophilic fibers are firmly anchored in the pore walls of the foam structure, and that the fiber amount is at least 10% by weight of the total weight of the open-cell foam in dry condition.
Foam materials made of traditional superabsorbent polymers (e.g. polyacrylic acid/polyacrylate polymers) are usually hard and stiff when dry, and inelastic when wet—they tend to fall apart under pressure. For these reasons, superabsorbent materials are usually included in absorbent articles in granular form.
It would therefore be advantageous to design a new absorbent material including superabsorbent polymers and cellulosic fibers with improved mechanical and gel properties in a swollen condition, and at the same time retain absorbent, spreading and storage properties. In particular, it would be useful to provide an absorbent material which has improved strength, yet which—at the same time—does not suffer from lack of flexibility and brittleness.